Slab rolling

ABSTRACT

When rolling a steel slab along its edges to reduce its width the head end of the slab may experience a larger percentage reduction than the mid portions of the slab. A similar situation, but to a less marked extent may occur at the tail end of the slab. To avoid this disadvantage workpieces are rolled in a vertical mill stand and the roll gap of the mill stand is automatically controlled during rolling. In the combination of a vertical mill stand and a horizontal mill stand in which slabs are successively edge and flat rolled an automatic control system is employed to control means for adjusting the roll gap of the vertical stands during the rolling of at least a terminal length of the slab adjacent an end so as to produce a width profile such that after subsequent horizontal rolling the width of the terminal length is not less than the width of the remainder of the slab.

Dowsing et al.

[ 1 Jan. 23, 1973 1 SLAB ROLLING [75] inventorsz jolin Dowsing,Sheffield, Kenneth Alfred Yeomangwor k soti. both of England [73]Assigneez Davy and UnitedEingineeFin g Co.

I Ltd., Sheffield, England [22] Filed: Sept. 23, 1970 [21] Appl. No.:74,594

[30] Foreign Application Priority Data UNITED STATES PATENTS 3,543,548l2/l970 Smith, Jr. ..72/7 3,468,145 9/1969 Yeomans... "72/12 3,296,513l/l967 Morton ..72/l0 3,08 l ,653 3/l963 Kincaid ..72/8

Primary ExaminerMilton S. Mehr Att0rneyHolcombe, Wetherill & Brisebois[57] ABSTRACT When rolling a steel slab along its edges to reduce itswidth the head end of the slab may experience a larger percentagereduction than the mid portions of the slab. A similar situation, but toa less marked extent may occur at the tail end of the slab. To avoidthis disadvantage workpieces are rolled in a vertical mill stand and theroll gap of the mill stand is automatically controlled during rolling.In the combination of a vertical mill stand and a horizontal mill standin which slabs are successively edge and flat rolled an automaticcontrol system is employed to control means for adjusting the roll gapof the vertical stands during the rolling of at least a terminal lengthof the slab adjacent an end so as to produce a width profile such thatafter subsequent horizontal rolling the width of the terminal length isnot less than the width of the remainder of the slab.

3 Claims, 2 Drawing Figures PATENTEDJAN23 I973 3 712 095 SHEET 1 [1F 2 ISLAB ROLLING This invention relates to the rolling of metal workpiecesand is particularly concerned with the preliminary edge rolling of asteel slab to reduce its width. I

The reductions taken by the edging stands of hot strip mills have beenin the past relatively small, larger reductions having been found toproduce bad edge shape in the eventual strip, after horizontal or flatrolling. As a consequence, slabs are normally stocked in the range ofwidths, having increments of one half inch widths, to accommodate thevariations in strip widths required. The cost of providing a stock ofslabs having so many different widths is substantial.

We have discovered that if a relatively large reduction is taken in avertical edger mill, there is a danger that, although the roll gap ofthe mill may be held nominally constant, the head end of the slab mayexperience a larger percentage reduction than the midportions of theslab. Although the increased reduction may be effective for less than afoot length of the slab, the result is the production of up to 100 feetof strip with a width smaller than that required. This narrower striplength must be discarded, or the remainder of the strip trimmed to thesmaller widths. A similar situation, but to a less marked extent, mayoccur at the tail end of the slab.

We believe that as the head end of the slab passes through the edgermill, there is relatively little resistance to metal movement in theforward direction and material is drawn forward between the rolls atroll gap width. As the slab progresses through the mill, initially thewidth of the part already drawn through the rolls is reducedby shearingforces communicated from the material then between the rolls. The effectdecreases as the slab progresses through the roll gap, and theresistance to deformation increases until, after about eight inches fromthe head end, the displacement of metal caused by the rolling operationappears largely as an increase in the slab thickness. The outcome isfirstly a width taper from the head end over about the first eightinches, and secondly the production of a dogbonesection in.the slabwhere the displaced metal is unablefto move-lengthwise. When the slab issubsequentlyrolled in a horizontal mill, the dog-bone section is rolledout, increasing the width of the slab again; the head end however isfree of the dog-bone section and does not therefore experience the samewidth enlargement. Consequently both the angle of the taper and theextent of the taper are increased.

In the horizontal rolling stands, a flare of the head end is producednaturally. This flare offsets to some extent the taper produced in thevertical mill stand. However, the length of material over which theflare occurs remains roughly constant while the workpiece is elongatedas it is reduced in thickness. The result is that the width at theextreme head end of a finished strip may be approximately the same asthat at mid-length, but there is a length of up to 100 feet of stripjust inside the head end which is narrower than either.

We have found that it is possible to prevent or reduce the production ofoff-widths slab by initially setting the rolls to a separation greaterthan that required for the main body of the slab and progressivelydecreasing the roll separation over the first part of the slab length.The variation in separation is chosen so as to compensate for the widthvariations mentioned above, in order to achieve afterhorizontal slabrolling substantially constant width or a head end with a width greaterthan that of the main body of the slab, since the strip head end maythen be trimmed to width at little expense. The

same procedure may be followed in reverse at the tail end.

Accordingly, one aspect of the present invention resides in a method ofrolling workpieces in a vertical mill stand in which the roll gap of themill stand is automatically controlled during rolling.

A second aspect of the invention resides in a method of rolling a metalslab in which the slab is edge-rolled in a vertical mill stand andsubsequently is flat rolled in a horizontal mill stand, and in which theroll gap of the vertical mill stand is automatically varied duringrolling of a terminal length of the slab adjacent an end, to obtain awidth profile such that, on subsequent flat rolling, the widths of thecorresponding terminal lengths is not materially less than the widths ofthe remainder of the slab. In particular, at the head end of the slab,the roll gap may be progressively reduced during the first part of theslab length.

Another aspect of the invention lies in the combination of a verticalrolling mill stand and a horizontal mill stand in which slabs aresuccessively edge-and flatrolled, means for adjusting the roll gap ofthe vertical stand, and an automatic control system for controlling theadjusting means during the rolling of at least a terminal length of theslab adjacent an end so as to produce a width profile such that, aftersubsequent rolling in the horizontal stand, the width of thecorresponding terminal length is not materially less than the width ofthe remainder of the stand.

By controlling the roll gap on the vertical mill stand by the invention,it is possible to take greater reductions in that stand than haspreviously been possible without producing unacceptable amounts of scrapin the rolled strip. As a consequence, a more limited number of slabwidths need be stored; thus, the stored slab width increment may beincreased from the half inch previously mentioned to 2 inches, therequired variation in strip widths being obtained by selection of theamount of reduction taken from the slabs. Reducing the number of stockwidth slabs results in considerable saving in cost.

Where, for example, a net reduction of slab widths of 1% inches isrequired from a standard slab width, in order to achieve a requiredwidth in the final strip, the vertical rolls may have to be set toreduce the slab widths by 3 inches or 3% inches, to allow additionallyfor the width regained by rolling down the dog-bone section in the nexthorizontal rolling pass. In such a case, the overall taper produced atthe head end might be about two inches extending over 12 to 20 inches ofthe slab length. To prevent the occurrence of this taper, the rolls ofthe vertical mill are initially set at two inches greater than that toachieve the reduction of 3 to 3% inches, and the roll gap progressivelydecreased over the first twelve to twenty inches of the slab length. Ifthe rolling speed is three feet a second, this reduction in roll gapmust be made in from 0.3 to 0.6 seconds. It must also be carried outagainst the rolling load which may be 300 tons.

As a rapid acting mechanism is required in order to adjust the roll gapquickly, the normal screw devices for setting the roll gap of verticalrolls must be replaced by rapid-acting screw adjusting mechanisms or,preferably. by replacing or supplementing the normal screw devices by ahydraulic actuator. However, other mechanism for setting and adjustingthe roll gap, such as wedges, and electric thrustors, may be employed inplace of the screws.

The invention will be more readily understood by way of example from thefollowing description of a vertical rolling mill stand and controlstherefor, reference being made to the accompanying drawings, in whichFIGS. 1 and 2 illustrate alternative forms of vertical mill stand withdifferent forms of control.

Referring to FIG. 1, the vertical mill stand is illustratedschematically by a pair of vertical rolls 12, 13 supported in theirrespective roll carriages 14, 15. The positions of the vertical rolls12, 13 are set by screws 16, 17, which are threaded into nuts 18, 20which are supported by the mill frame and which are driven by anelectric motor (not shown).

lnterposed between the screws 16, 17 and the roll carriages l4, 15 aretwo hydraulic short-stroke capsules 21, 22. Springs 23 act on the rollcarriages 14, 15 and bias them in directions tending to open the rollgap. These springs take up back-lash in the screws 16, 17 and provideforces to return the pistons of the capsules 21, 22 when the pressure ofthe hydraulic supply is withdrawn; the springs 23 may be replaced bypullback cylinders providing a small constant force suffrcient toovercome friction.

A roller table, exemplified by the roller 24 is arranged to leadworkpieces (the slab 25), into and out of the vertical mill.

The screws 16, 17 are initially set to a value equal to the settingrequired to achieve the desired reduction of the slab widths, plus theamount of width taper that would be achieved, if the roll gap was heldfixed throughout the length of the slab. Thus, in the example previouslygiven, the screws 16, 17 are set to give a roll gap two inches greaterthan that required for the desired reduction. During the rolling of thefirst 12 to inches of the slab length, the roll gap is decreased byoperation of the cylinders 21, 22, so that at the end of that length,the roll gap is that required for the desired reduction. The operationof the capsules 21, 22 is controlled by the control system shown in thelower half of FIG. 1.

The control system is arranged to cause the roll gap between rolls 12,13 to be decreased at a constant rate. The two capsules 21,22 areconnected through lines 26 to a cylinder 27, having a piston 28. Thepiston 28 is connected by its rod 30 to a rack 31, which meshes with apinion 32 driven by a motor 33. As the slab approaches the edging rolls,its leading edge is detected by a photocell (not shown) and, after adelay appropriate to the distance of the photocell from the roll biteand the speed of the roller table 24, the adjusting motor is switchedon. The motor drives the rack 31 from right to left as viewed in H6. 1and liquid is forced out of the cylinder 27 through the lines 26 andinto the capsules 21, 22 to cause a decrease in the roll gap. Thecross-sectional area of the cylinder 27 is less than the sum of theareas of the capsules 21, 22 so as to effect a force amplification.

The piston rod 30 carries collars 34, 35 which cooperate with a fixedstop 36 to set the extreme limits of movement of the piston 28 andtherefore of the capsules 21, 22. The collars 34, 35 are set at aseparation which, taking into account the force amplification, achievesthe required reduction in roll gap during the rolling of the head end ofthe slab. Similarly, the speed of the motor 33 is set so that the piston28 is driven from one extreme limit to the other in the time for thehead of the slab to enter the rolls by twelve to twenty inches.Preferably, the fixed stop 36 takes the form of a limit switch which isactuated by the collars 34, 35 to de-energize the motor 33 at the endsof the travel.

Instead of having the motor 33 rotating at constant speed, its speed maybe controlled in accordance with the varying value of the roll gapaccording to a preselected pattern. For this purpose, a transducer,shown as a potentiometer 37 may be coupled to the rack 31 and its signalemployed to control the motor speed control circuit 38 of motor 33.

Again, the motor 33 may be controlled by a closed loop system inaccordance with the position of the slab 25 in relation to the roll gap.For this purpose, a slab position transducer, such as a scanning typephotocell located above the roll gap, is provided to give a signalrepresenting the position of the head of the slab as the head approachesand enters the roll gap. The resulting position signal is compared withthe signal from the rack transducer 37 and the motor 33 controlledaccordingly, in order that, at each position of the slab 25 within theroll gap, the roll gap has a given preselected magnitude.

It will be appreciated that, although two capsules 21, 22 have beenillustrated and described, it is possible to employ a single capsule,which in that case must have a larger travel than either of the twoillustrated. It will also be understood that, although the operation hasbeen described in relation to the reduction in roll gap at the entry ofthe head end of the slab, a similar operation may be effected as thetail end passes through the roll gap, the roll gap being increased overthe last terminal length of the slab; the control system is asillustrated and operates in reverse to the procedure described.

In FIG. 2 the screws 16, 17 of FIG. 1 are dispensed with and thefunction of the screws 16, 17 and capsules 21, 22 is performed by pistonand cylinder assemblies 40, 41 arranged between the mill frame and thecarriages 14, 15.

Each of the assemblies 40, 41 has associated with it a positiontransducer 42, 43 giving, on lines 44, 45 signals representing thepositions of the roll carriages 14, 15 in relation to the mill housing.The cylinder of each of the assemblies 40, 41 is supplied with liquidunder pressure through respective pilot valves 46, 47 and lines 48, 50.Each pilot valve is controlled by the output of one of two amplifiers51, 52 which receive position signals from the respective transducers44, 45 respectively and also receives signals representing the requiredroll gap setting. In the amplifiers, the required setting is comparedwith the actual setting as represented by the signals on lines 44, 45and the pilot valves 46, 47 controlled to maintain the two in equality.

The datum signals representing the required roll gap setting are derivedon lines 53, 54, which are connected in parallel to the amplifiers 51,52, and which are connected respectively to a manually controlled device55 for setting the gap required during the rolling of the mid portionsof the slab, and from a generator 56 which gives a signal which variesat the speed of the required change in roll gap during the rolling ofthe slab end. The generator 56 is controlled by an initiating device 57which receives signals on line 58 from a photocell detecting theapproach of the slab, and a line 60 from a timing mechanism such thatthe device 57 initiates the generator 56 as the head end enters the rollgap. Generator 56 is a potentiometer or like device which generates aramp voltage which decreases over the time taken by the first 12 toinches of the slab to pass through the roll gap and by an amount neededto reduce the roll gap from the initial value to the value set by device55. Alternatively, a closed loop control system may be employed, thesignal on line 60 being derived from a slab position transducer, thegenerator 56 then giving a signal to amplifiers 51, 52, which signalvaries in a prescribed pattern with the position of the slab within theroll gap.

Whatever control system is employed, the roll gap is decreased over thehead end of the slab to compensate for the taper in the head end thatwould otherwise be produced during the vertical rolling and subsequenthorizontal rolling. As a result, the strip rolled from the slab does nothave an appreciably smaller width at the head end than in the mid lengthand no trimming of the mid length is required. The same operation occursat the tail end, again with the aim of achieving a strip ofsubstantially constant width along its length.

We claim:

1. A method of rolling a metal slab which comprises the steps of firstedge rolling said slab in a vertical mill stand while progressivelydecreasing the roll gap of said vertical mill stand as a first portionof said slab passes therethrough to impart a selected width profile tosaid first portion, maintaining said roll gap substantially constantduring the edge rolling of at least the major part of the remainder ofsaid slab, subsequently flat rolling said slab in a horizontal millstand, and so selecting said width profile that after said subsequenthorizontal flat rolling the width of said first portion of said slab isat least as great as that of said major part of said slab.

2. A method as claimed in claim 1 which also comprises the step ofincreasing said rolling gap as the last portion of said slab passesthrough said vertical mill stand so that after said subsequenthorizontal flat rolling the width of said last portion of said slab isat least as great as that of said major part.

3. A method of rolling a metal slab as claimed in claim 1 in which saidroll gap is decreased at constant speed.

1. A method of rolling a metal slab which comprises the steps of firstedge rolling said slab in a vertical mill stand while progressivelydecreasing the roll gap of said vertical mill stand as a first portionof said slab passes therethrough to impart a selected width profile tosaid first portion, maintaining said roll gap substantially constantduring the edge rolling of at least the major part of the remainder ofsaid slab, subsequently flat rolling said slab in a horizontal millstand, and so selecting said width profile that after said subsequenthorizontal flat rolling the width of said first portion of said slab isat least as great as that of said major part of said slab.
 2. A methodas claimed in claim 1 which also comprises the step of increasing saidrolling gap as the last portion of said slab passes through saidvertical mill stand so that after said subsequent horizontal flatrolling the width of said last portion of said slab is at least as greatas that of said major part.
 3. A method of rolling a metal slab asclaimed in claim 1 in which said roll gap is decreased at constantspeed.